A vibration-type actuator is a non-electromagnetic actuator configured to generate a high-frequency vibration in an electro-mechanical energy conversion element connected to an elastic element by applying an alternating voltage to the electro-mechanical energy conversion element such that vibration energy is output in the form of continuous mechanical motion.
The vibration-type actuator is used, for example, in autofocus driving in a camera. In the autofocus driving, a high-accuracy positioning control is necessary. To this end, a position feedback control using a position sensor is performed. In a control circuit that performs the feedback control, a control parameter is calculated based on a deviation of a position signal detected by the position sensor from a position command value.
It is possible to control a speed of the vibration-type actuator by adjusting a control parameter such as a driving frequency, a phase difference between two-phase driving voltage signals applied to an electro-mechanical energy conversion element of a piezoelectric element or the like, a driving pulse width, or the like. For example, a greater exciting amplitude is obtained by setting the driving frequency closer to a resonance frequency of the piezoelectric element, and thus it is possible to drive a lens, which is a driving target object, at a high speed. A driving frequency signal with a rectangular wave form is generated by a pulse generator based on the control parameter described above, and the driving frequency signal is stepped up to a particular alternating voltage by the driving circuit. It is possible to control the positioning of the vibration-type actuator by repeatedly applying the alternating voltage to the piezoelectric element while controlling the control parameter.
The driving circuit is described in detail below. The driving circuit has a function of stepping up a rectangular wave signal input from the pulse generator by a factor of several to several tens by using a coil, a transformer, or the like and outputting a resultant sinusoidal alternating voltage. The signal of the pulse generator is on/off-controlled by a switching circuit in terms of the frequency and the pulse duty and adjusted according to the driving frequency.
The driving circuit using the coil is an LC step-up circuit configured to electrically amplifying a signal with a particular frequency by using LC resonance of inductance of a coil and capacitance of the piezoelectric element. The step-up ratio thereof is generally in a range of about 1.5 to 3. The step-up circuit using the transformer is used when a further larger step-up ratio is required. The step-up ratio thereof can be freely adjusted by a turn ratio between a primary winding coil and a secondary winding coil of the transformer. In general use, the step-up ratio is in a range of about 3 to 30. In many cases, the step-up circuit using the transformer is used when the vibrating element needs a large driving voltage in a range of about 100 to 500 Vpp. In the transformer step-up circuit, by providing a coil element in series to the piezoelectric element on the primary side or the secondary side of the transformer, it is possible to more effectively remove harmonic components in generating the sinusoidal wave.
A conventional technique associated with a circuit using a transformer for driving a vibration-type actuator is described below.
PTL 1 discloses a piezoelectric vibrating element driving circuit configured to drive a piezoelectric vibrating element at a frequency close to a resonance frequency by using a one-transistor switching circuit and a transformer. By disposing a coil with a proper inductance on the secondary side of the transformer such that the coil is connected in series to the piezoelectric vibrating element, a nearly sinusoidal wave is obtained in the voltage and the current of the piezoelectric vibrating element, and, according to the phase of its current wave form, the driving frequency is controlled.
PTL 2 discloses a vibration motor in which an inductance element is connected to a driving electrode of the vibration motor, and a capacitance element is connected in parallel to an equivalent circuit of the motor such that an optimized motor performance is obtained. As an example, a driving circuit using a transform is disclosed.
PTL 3 discloses a driving circuit such as that described below. That is, the driving circuit for an ultrasonic motor disclosed in PTL 3 includes an oscillator which is a reference-frequency oscillation circuit and which generates a high-frequency signal for driving the ultrasonic motor, and buffer elements that receive the high-frequency signal from the oscillator and drive switching transistors in the driving circuit. The driving circuit further includes full-bridge switching transistors that turn on/off the conduction of the high-frequency signal via the buffer elements, a transformer for stepping up a low voltage of driving high-frequency power, and a coil for shaping a waveform output from the transformer.
PTL 4 discloses a driving circuit in which variable control means is disposed for controlling a duty ratio of high-frequency power to be applied to a piezoelectric vibrating element serving as a rotation driving source of an ultrasonic motor thereby compensating for a change in speed caused by a change in load, which makes it unnecessary to use a chopper circuit (DC-DC conversion circuit) for controlling the voltage. That is, in the circuit disclosed, power-isolated-type half bridges are combined so as to function as an inverter circuit that supplies high-frequency power to the piezoelectric element. A pulse output from the inverter circuit is applied to the piezoelectric element via a step-up transformer. Frequency control means is provided to correct a fluctuation caused by a disturbance on a load such that the high-frequency power applied to the piezoelectric element is fed back via a step-down transformer to variably control the output frequency of the inverter circuit. Furthermore, in the disclosed technique, based on the output from the frequency control means and a voltage detected by means of detecting a vibration state of the piezoelectric element, the width of a rectangular wave pulse is varied by pulse width control means and the resultant output is input to the inverter circuit. In the description of the disclosure, when a reduction in the rotation speed occurs, the pulse width of the rectangular wave is increases, but when an increase in the rotation speed occurs, the pulse width of the rectangular wave is reduced.
PTL 5 discloses a driving circuit including an oscillator that outputs a reference signal in the form of a pulse train, and a frequency tracking circuit that controls the frequency of the reference signal output from the oscillator so as to be equal to a driving frequency optimum for the ultrasonic motor. This driving circuit further includes a photosensor that detects a rotation speed of a rotation part of the ultrasonic motor, and pulse width modulation means for modulating the pulse width of the reference signal such that the rotation speed detected by the photosensor becomes equal to a target rotation speed. This driving circuit further includes a power amplifier that supplies a drive signal with an amplitude corresponding to the modulation of the pulse width provided by the pulse width modulation means to an electrode of the piezoelectric element. The driving circuit disclosed is capable of changing the amplitude of the ultrasonic motor driving voltage output from the power amplifier, i.e., the step-up transformer by modulating the pulse width of the reference signal thereby accurately controlling the rotation speed. Thus, this driving circuit disclosed is capable of controlling the pulse width and the oscillation frequency by using the output from the photosensor and the feedback signal of the frequency tracking circuit thereby precisely and accurately controlling the rotation speed of the ultrasonic motor.
As described above, driving circuits using a transformer for controlling a vibration-type actuator and control techniques thereof have been proposed.